1. Field of Invention
The present invention relates to an improved cooling system for germanium radiation detectors. More specifically, the present invention combines the use of an innovative new electromechanical cryogenic cooler with novel cryogenics to produce a low-cost, extremely reliable germanium gamma-ray system.
2. Description of the Related Art
Gamma-ray detectors in the form of large germanium diodes have been the preferred detectors for use in high resolution gamma-ray spectroscopy for many years. Germanium gamma-ray detectors are very accurate, having the capability of measuring the energy of a 1 MeV gamma-ray to better than 0.1 percent accuracy. However, the signal produced by these detectors is very small. For example, it is known that a 1 MeV gamma-ray produces a charge signal of only 5.4 femtoCoulombs of charge. Integrated on a typical detector capacitance of 20 picoFarads the resulting voltage signal is 3 mV. In order to preserve the intrinsic accuracy of the detector, all noise sources must be in the microvolt range. Achieving such a low noise requires a highly controlled environment for the detector.
At room temperature the dark current in a germanium diode produces noise far larger than the signal itself. When the operating temperature of the detector is lowered to cryogenic temperatures, typically about 100 Kelvin, the dark current is reduced to an insignificant level. Because the germanium detector signal itself is highly temperature dependent, the temperature must be held very stable. A temperature change of only 1 degree will cause an error of 0.2 keV in the 1 MeV gamma ray measurement.
Since the germanium detector is reverse biased to several thousand volts in operation, it is necessary that the detector surfaces be kept in a highly insulating state. This normally requires a very clean vacuum environment, free of condensable gases that could cause noisy leakage currents in the detector.
The combination of high reverse bias voltages and very small signals results in a sensitivity to microphonically generated noise. The environment must thus be free of vibrations at frequencies in the pass band of the spectroscopy amplifier system.
It is well known to use liquid nitrogen (LN) cooling for achieving this highly controlled environment. The temperature of LN at atmospheric pressure is about 77xc2x0 K. and is quite stable. U.S. Pat. No. 4,851,684 issued to G. N. Martin et al., fully incorporated herein by reference, discloses a photon detector system including a vacuum-jacketed radiation detector in a cryostat assembly. In the ""684 patent, a cryogenic gamma radiation detector cooled by a dewar is specifically disclosed. A germanium detector is enclosed in a vacuum insulated cryostat in thermal communication with a reservoir of LN. Martin et al. disclose a particular form of such a cryostat offering the additional advantage of allowing the cryostat to be conveniently separated from the source of cooling.
LN based cooling systems are relatively inexpensive and reliable. They do, however, require periodic refilling of the LN, which may present problems in remote installations or hazardous environments. The filling itself can be a safety hazard and requires a trained operator. Alternative methods of cooling have been available for many years but have not been widely used because of a number of problems. Mechanical coolers based on the Stirling cycle have been used in military and space systems but have a prohibitively high cost and must be periodically maintained, requiring the germanium detector to be removed from service. Recently, a class of mechanical coolers based on principles similar to the air conditioner has been developed. These coolers use a mixed-refrigerant throttle cycle (MRTC) to produce cooling. Germanium spectroscopy systems using these systems are now commercially available but are still much more expensive and less reliable than LN based systems.
U.S. Pat. Nos. 5,617,739 issued to Little and 5,724,832 issued to Little et al., both fully incorporated herein by reference, disclose a unique version of the MRTC cooler including a novel self-cleaning feature to allow the use of an inexpensive mass-produced air conditioning compressor in the system. The oil clogging which would be expected to result at cryogenic temperatures is prevented by the self-cleaning feature. The result is an inexpensive, highly reliable cooler.
However, the above references fail to disclose a highly reliable cooling system for germanium detectors in order to overcome the sensitivity of the detector to microphonically generated noise as a result of high reverse bias voltages and very small detector signals.
Accordingly, it is an object of this invention to provide a low-cost, high-performance, and highly reliable cooling system for germanium detectors.
It is a further object of the present invention to provide a germanium detector operating environment that meets all the requirements for optimum performance of such detectors incorporating said cooling system.
A germanium gamma-ray detector contained in a vacuum insulated cryostat is provided. The present invention provides a low-cost, high-performance, and highly reliable cooling system for germanium detectors. Moreover, the present invention provides a germanium detector operating environment that meets all the requirements for optimum performance of such detectors incorporating said cooling system.
A self-cleaning MRTC cooler is incorporated in the present invention. The MRTC cooler includes a counter-current heat exchanger which is received within a cooler housing.
A removable cryostat is provided for being carried by the cooler housing. A connector tip is machined to a precise diameter to mate with the cooler. A capsule cold finger provides the cooling path to germanium detector element. A centering spacer/isolator is provided for maintaining the position and supporting the weight of the detector in an end cap without conducting an excessive amount of heat into the detector. A capsule flange is provided to substantially close the volume within the end cap. A thin vacuum wall is disposed between the capsule flange and the connector tip in order to accomplish a vacuum within the end cap when attached to a cooler.
The heat exchanger and the throttle capillary of the cooler cool the cold block. A thermal link is thermally connected to a cooler cold finger, which has an internal diameter precisely matched to the diameter of the capsule cold finger. The cooler cold finger is connected to the warm outer wall by a vacuum isolator. At room temperature, the cooler cold finger and the detector capsule connector tip are configured to accomplish a close fit. However, at cryogenic temperatures, the fit is extremely tight and provides a high thermal conductivity joint. A gas adsorber, typically a molecular sieve, is received within the cooler cold finger. A threaded bayonet mates with the detachable cryostat and completes an insulating vacuum space for the cooler section.
The cold block incorporates a heater and temperature sensor to accurately control the temperature of the germanium detector. An external temperature controller is provided for monitoring the temperature sensor and modulating the heat input into the heater to maintain the proper temperature for optimum performance of the germanium detector. The cold block also provides a container for cooling a gas adsorber, typically a molecular sieve, for maintaining the required vacuum level. The adsorber provides the insulating vacuum only in the cooler section.